1. Field of the Invention
The present invention relates to a water impervious rubber or plastic insulated power cable, and more particularly, to a water impervious insulated power cable, which is capable of exhibiting excellent water imperviousness for a long period of time, compared with a conventional water impervious rubber or plastic insulated power cable.
2. Prior Art
A typical rubber or plastic insulated power cable (hereinafter referred to as a "power cable") comprises a metal shield layer and a corrosion-protective layer (sheath layer) covering a cable core, which has a conductor, an extruded shield, a rubber or plastic insulation layer and an extruded insulation shield; the last three members being sequentially formed around the conductor. In this power cable, it is important to protect the cable core from the entry of water in the external environment of the cable.
As a conventional power cable provided with waterproof measures, there is one comprising a metal-plastic laminate tape, which is longitudinally applied under a corrosion-protective layer so that its plastic surface is positioned on the corrosion-protective layer side, and the plastic surface and the corrosion-protective layer being made into one united body to form a water impervious layer as disclosed under U.S. Pat. No. 4,472,597.
In the power cable provided with such a water impervious layer, however, there is the danger that, if the corrosion-protective layer should be damaged due to some cause or other, the water impervious layer, which is made integral with the corrosion-protective layer, will easily be affected by the damage. For this reason, it is an unavoidable problem that the water impervious property of the power cable deteriorates if the corrosion-protective layer incurs a damage.
To solve the aforementioned problem, the inventor has completed an invention on a power cable, wherein a laminate tape consisting of a metal tape and an electrically conductive plastic tape is used as the water impervious layer tape in the above-mentioned structure, and the laminate tape is longitudinally applied so that its electrically conductive plastic layer faces the cable core, thereby forming the electrically conductive plastic layer and an insulation shield into one body (refer to U.S. Pat. No. 4,626,619).
Further, the inventor has completed a power cable having a structure, as shown in FIG. 1, consisting of a conductor shield 2, an insulation layer 3, an insulation shield 4, and a water impervious layer 5 formed in sequence around conductors 1, and a semiconductive fabric tape-wrapped layer 6 formed by lapping a semiconductive fabric tape, which consists of an electrically conductive rubber applied to a woven fabric which has a good heat cycle following property and a memory rate of expansion and contraction thereof is 30% or more, around the water impervious layer 5 (U.S. Pat. No. 4,703,134).
When a power cable is in actual use, the power cable is subjected to frequent repetition of energizing and de-energizing. As a result, the power cable is continually subjected to heat cycles wherein the temperature of the whole cable including the cable core rises during the energizing period, while the temperature drops during the de-energizing period. Therefore, the cable core, the metal shield layer and the like of the power cable naturally repeat expansion and contraction due to the heat cycles. The repetition of the expansion and contraction adds to the fatigue of and damage (cracks or the like) to the composing materials, especially the water impervious layer, or the power cable, significantly leading to a shortened service life of the product.
In the power cables described in the specifications of U.S. Pat. Nos. 4,559,973 and 4,703,134, the water impervious layer and the cable core are made into one united body; therefore, the fatigue of and damage to the water impervious layer (mainly the metal tape) are increased by the repetitive expansion and contraction of the cable core caused by the heat cycles.
The power cable described in the specification of U.S. Pat. No. 4,703,134 has a special semiconductive fabric tape-wrapped layer with a good heat cycle following property provided between the water impervious layer and the metal shield layer; therefore, the damage to the water impervious layer from the expansion and contraction of the cable core and the metal shield layer can be significantly reduced.
In this power cable, however, as is obvious from FIG. 1, an edge portion of the semiconductive fabric tape constituting the semiconductive fabric tape-wrapped layer 6 forms discontinuity portion in terms of configuration and also the pressure applied to the water impervious layer 5. As a result, the metal tape constituting the water impervious layer 5 becomes susceptible to fatigue and damage at the discontinuity portion during the heat cycles. In addition, during the aforesaid heat cycles, the semiconductive fabric tape-wrapped layer 6 and the water impervious layer 5 rub against each other, contributing also to the fatigue of and damage to the water impervious layer 5.
Thus, the power cables proposed by the inventor previously both provide excellent characteristics as power cables, however, they still have room left for improvement in respect of the deterioration of water impervious property caused by the fatigue of and damage to the composing material primarily due to the heat cycles.